A self-propelled anti-aircraft weapon (SPAA, also self-propelled air defense, SPAD, or self-propelled anti-aircraft gun, SPAAG) is an anti-aircraft gun or surface-to-air missile launcher mounted on a mobile vehicle chassis. The Russian equivalent of SPAAG is ZSU, for zenitnaya samokhodnaya ustanovka, ("anti-aircraft self-propelled mount").
Specific weapon systems include machine guns, autocannon, larger guns, or missiles, and some mount both guns and longer-ranged missiles. Platforms used include both trucks and heavier armored fighting vehicles such as APCs and tanks, which add protection from aircraft, artillery, and small arms fire for front line deployment.
Anti-aircraft guns are usually mounted in a quickly-traversing turret with a high rate of elevation, for tracking fast-moving aircraft. They are often in dual or quadruple mounts, allowing a high rate of fire. Today, missiles (generally mounted on similar turrets) have largely supplanted anti-aircraft guns.
First World War
Anti-aircraft machine guns have long been mounted on trucks, and these were quite common during World War I. A predecessor of the WW2 German "88" anti-aircraft gun, the WWI German 77 mm anti-aircraft gun, was truck-mounted and used to great effect against British tanks.
The British QF 3 inch 20 cwt was mounted on trucks for use on the Western front.
Between the wars the British developed the Birch gun, a general purpose artillery piece on an armoured tracked chassis capable of maintaining formation with their current tanks. The gun could be elevated for anti-aircraft use.
Among early pre-war pioneers of self-propelled AA guns were the Germans. By the time of the war, they fielded the SdKfz 10/4 and 6/2, cargo halftracks mounting single 20 mm or 37 mm AA guns (respectively). Later in the war similar German halftracks mounted quadruple 20 mm weapons.
Second World War
Larger guns followed on larger trucks, but these mountings generally required off-truck setup in order to unlimber the stabilizing legs these guns needed. One exception to this rule was the Italian Cannone da 90/53 which was highly effective when mounted on trucks, a fit known as the "autocannoni da 90/53". The 90/53 was a feared weapon, notably in the anti-tank role, but only a few hundred had been produced by the time of the armistice in 1943.
Other nations tended to work on truck chassis. Starting in 1941, the British developed the "en portee" method of mounting an anti-tank gun (initially a 2 pounder) on a truck. This was to prevent the weapon from being damaged by long-distance towing across rough, stony deserts, and it was intended only to be a carrying method, with the gun unloaded for firing. However, crews tended to fire their weapons from their vehicles for the mobility this method provided, with consequent casualties. This undoubtedly inspired their Morris C9/B (officially the "Carrier, SP, 4x4, 40 mm AA"), a Bofors 40 mm AA gun mounted on a chassis derived from the Morris "Quad" Field Artillery Tractor truck. Similar types, based on 3-ton lorries, were produced in Britain, Canada and Australia, and together formed the most numerous self-propelled AA guns in British service.
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